When a machining process for repeating acceleration and deceleration of a spindle motor in one cycle is repeatedly executed by a machine tool a plurality of times, in some case, the temperature of the spindle motor rises and finally the spindle motor overheats and urgently stops.
To deal with this problem, Patent Literature 1 discloses a technology for predicting a motor temperature from a motor temperature measured by a temperature sensor. According to the technology of Patent Literature 1, as measures against a likelihood that the predicted motor temperature is equal to or higher than a threshold to cause overheating, a method of reducing maximum speed of a motor, a method of increasing an acceleration/deceleration time constant of the motor and reducing the value of a motor current during acceleration and deceleration, and a method of increasing an operation stop time when a machine driven by the motor repeatedly performs the same operation pattern, and the like.
According to the technology of Patent Literature 1, the prediction of the motor temperature is performed on the basis of a relation between a motor average current obtained by measurement in advance and a motor temperature rise amount or the prediction of the motor temperature is performed using a function calculated in advance from an experiment data or the like for calculating a predicted motor temperature from a measured motor temperature difference in a predetermined cycle period. Changes of the motor acceleration/deceleration time constant, the motor maximum speed, and the operation stop time are adjusted to values set in advance.